Conventionally, a 3-roll drive type pipe molding apparatus used as a reducer for urging to mold the electro-resistance-welded pipe smaller in diameter, is proposed and well-known by the same applicant, as disclosed in, for example, the Japanese Patent Laid-Open Gazette No. Hei 3-294008. Also, a 4-roll molding apparatus, in which an upper roll and a lower roll only are driven and a left roll and a right roll are idling, is of well-known technique and disclosed in, for example, the Japanese Patent Laid-Open Gazette No. Hei 5-212440.